Water dispersible, low molecular weight polyamide resin particles of uniform sizes, method of preparing same and coatings formed therefrom

ABSTRACT

Finely-divided, water dispersible, low molecular weight, linear polyamide resin particles having a unique morphology and of uniform particle sizes are formed by preparing a linear polyamide of a predetermined low molecular weight in the presence of water followed by rapidly quenching the reaction mass with an aqueous medium below the freezing point of the polyamide and continuing the cooling of the mass to a temperature sufficiently low so as to prevent particle growth and structural alteration while regulating the pH of the quenched mass to provide particles of predetermined, uniform size dispersed in the aqueous medium. The particles vary from ultimate flaky sheets or lamellae to loosely packed, randomly oriented, clusters of flaky sheets. Spray dried products may be agglomerates which disintegrate readily in water to the original particles or aggregates depending upon intended uses. The dispersions and dried products are useful in the coating art and the polyamide may be polymerized to high molecular weights after application to a substrate.

This is a continuation of the now abandoned application Ser. No.492,609, filed July 29, 1974 which in turn is a division of applicationSer. No. 248,063, filed Apr. 27, 1972, now U.S. Pat. No. 3,844,991.

This invention relates to low molecular weight, linear polyamide resinsin the form of finely-divided, water dispersible particles of sizesfalling within controlled, very narrow size ranges, aqueous dispersionsthereof and a method of preparing same.

Finely divided polyamide resins have been proposed for use in thecoating art. In powder form, they have been used by immersing a heatedarticle in a bed, such as a fluidized bed, of the powder, or by a flamespraying procedure. In British Pat. No. 1,180,023 it is proposed toimmerse the heated metal article into a fluidized bed of finely divided,low molecular weight polyamide particles containing an oxy-acid ofphosphorus and subsequently subjecting the article to a further heattreatment so as to further polymerize the polyamide and thereby form acoating of a high molecular weight polyamide. Such method may besatisfactory for coating of rods and simple shapes but is not adaptableto the coating of complicated shapes or large articles, or articles orobjects or low heat capacity such as, for example, wire screening, metalfoil, etc. Alternatively, it has been proposed to use solutions ofsuperpolyamides, however, the polyamides are generally soluble only intoxic and/or corrosive liquids such as phenol, cresol, formic acid,hydrochloric acid, or a limited number of organic liquids such asfurfuryl alcohol or formamide. These solutions require special handlingtechniques and are highly disadvantageous in that normally the solventpresents pollution problems and must be recovered.

As a further alternative, as, for example, in wire coating thesuperpolyamide resin may be in a molten state and the wire drawn throughthe molten resin. Both the solution and the molten techniques areundesirable because, in general, the synthetic, linear superpolyamidesare degraded rather rapidly at the solution and at the moltentemperatures which are required. Further disadvantages are associatedwith these methods of application utilizing the present commerciallyavailable high molecular weight polyamides. In the application of acoating from a solution of the polyamide, the coating is limited to afew tenths of a mil and, in general, several coatings are required withthe necessity of drying and fusing each coat before the application ofan additional coat. On the other hand, coatings applied by a moltentechnique are at least 10 mils in thickness because of the highviscosity of the molten polyamides.

Various methods have been proposed to form polyamide dispersions adaptedfor use in the coating art. The most common proposal is to dissolve thesuperpolyamide in a suitable solvent and then pouring the solution intoa larger volume of a non-solvent with vigorous agitation. It has alsobeen proposed to vigorously agitate water and a dispersing agent attemperatures sufficiently high to a melt a thermoplastic resin such as apolyamide and thereby form a dispersion of the resin. In these variousprior art procedures the polyamides are the so-called superpolyamideswhich are fiber-forming and film-forming high molecular weight polyamideresins.

In U.S. Pat. Nos. 3,299,011 and No. 3,536,647, a fiber- and film-formingsuperpolyamide is partially degraded so as to remove amorphous portionsof the polyamide and the resulting partially degraded polyamide issubjected to mechanical attrition in the presence of a liquid swellingmedium. The resulting product, which has been termed a microcrystalline,synthetic, linear polyamide, is dispersible in water and other liquidswelling media. As a result of the required mechanical attrition, thesize of the particles extend over a very wide range. Thus a severelyattrited product containing about 90% by weight of particles notexceeding 1 micron contains particles as small as 0.01 micron and aslarge as about 15 microns. A moderately attrited material will containparticles as large as 100 microns and particles under 0.2 micron andsome as small as 0.01 micron. Because of the presence of the minuteparticles, particularly those under 0.1 micron, upon drying, particlesbecome bonded together into larger size particles or agglomerates tosuch a degree that it is almost impossible to reduce the particles totheir original size and thus redisperse the material.

One of the characteristics of these prior dispersible polyamideparticles in their colloform or globular, granular form or shape.Because of this form or shape of particle which becomes deposited on thesubstrate being coated it is necessary to subject the substrate bearingthe deposited particles to an elevated temperature for a sufficientperiod of time so as to permit the complete melting and flowing of themolten particles to form a continuous coating.

One of the purposes of the present invention is to provide low molecularweight, linear polyamides in the form of finely-divided, waterdispersible particles of sizes falling within controlled, very narrowsize ranges by a simple and inexpensive method.

A further purpose of the present invention is to provide a simple andinexpensive method for the preparation of aqueous dispersions of thesefinely divided, low molecular weight, linear polyamide resins.

A further purpose of the present invention is to provide finely divided,low molecular weight, linear polyamide resin particles of a unique shapeor form as will be described hereinafter.

Another purpose of this invention is to provide a simple and inexpensivemethod for the production of finely divided, low molecular weight,linear polyamide resin particles of the unique shape or form which areextremely uniform in particle size.

Another purpose of this invention is to provide finely divided, lowmolecular weight, linear polyamide resin particles which are readilydispersible in water.

A further purpose of this invention is to provide aqueous dispersions offinely-divided, low molecular weight, linear polyamide resin particleswithin very narrow size ranges which may be applied directly to asubstrate by any desired conventional method without the necessity ofutilizing an intermediate or primer coating.

Another object of the invention is to provide finely-divided, lowmolecular weight polyamide resin particles which may be utilized inconventional powder coating of substrates so as to provide adherentcoatings without the necessity of utilizing an intermediate or primercoating.

Other objects and advantages of the invention will become apparent fromthe following description and the claims.

The present invention contemplates the preparation of low molecularweight, linear polyamide resin particles wherein water and a monomer ora mixture of monomers, or, alternatively, water and a high molecularweight, linear superpolyamide resin is heated in a closed or sealedsystem to a temperature above the melting point of the specificpolyamide which is being formed followed by quenching rapidly in anaqueous medium to a temperature below the freezing point of the formedpolyamide so as to obtain the unique particles of sizes and structure aswill be described hereinafter and continuing to reduce the temperatureof the mass to a point where particle growth is prevented and theinitial particle size and structure is retained while vigorouslyagitating or stirring the entire mass and controlling or regulating thepH of the entire mass. The resulting product consists of a stableaqueous dispersion of the low molecular weight polyamide particles. Theconsistency will vary depending upon the solids content generallyvarying from a milk-like dispersion at low solids content to apaste-like product at high solids content. When practicing the method ina batch-wise manner, as illustrated in the examples, the rapid quenchingand cooling may be effected conveniently under normal atmosphericconditions of pressure by discharging the heated mass into an open tankcontaining a sufficient quantity of a cold aqueous medium so that thetemperature of the entire quench mass is brought to a temperature belowthe boiling point of water, as, for example, a temperature not exceedingabout 90° C to 95° C.

One of the unique characteristics of the products is the uniformity ofsize of the low molecular weight polyamide particles achieved by acontrol of the pH value of the quenched mass; that is, the particles arewithin a very narrow range of particle sizes of not more than severalmicrons. For example, in those instances where the polyamide resin is alow molecular weight polycaprolactam and the pH of the final quenchedmass is above about 7 and up to about 9, or where an alkaline materialhas been added, if necessary, to provide a desired predetermined pHvalue within this range, the mean size of the particles will be withinthe range of between about 2 microns and about 6 microns. Where the pHof the final mass is about 3.5 to about 7, the particles will be of amean size of about 0.5 micron to 2 microns. As an alternative, a soapsuch as sodium, potassium or ammonium oleate, stearate, etc., may beincluded in the initial charge and in such instance where the quenchedmass has a pH of about 8.5, the particles will have a size between about0.1 micron and 0.5 micron. In those instances where a soap is presentand the pH of the quenched mass is about 3, all of the particles areappreciably below 0.1 micron in maximum size and of approximately thesame size, approximately 0.03 × 0.03 × 0.005 micron, a ratio ofthickness to maximum dimension of approximately 1 to 6.

The monomers satisfactory for the purpose of the present inventioninclude, for example, ω-aminocarboxylic acids, such as 6-aminocaproicacid, or their corresponding lactams or cyclic amides, such as,ε-caprolactam, and salts of diamines with dicarboxylic acids, such as,hexamethylene diamine with hexanedioic (adipic) acid, and mixtures ofω-aminocarboxylic acids, such as, 6-aminocaproic acid and11-aminoundecanoic acid, or ε-caprolactam and 12-aminododecanoic acid,and mixtures of ω-aminocarboxylic acid or lactam and a salt of a diaminewith a dicarboxylic acid, such as, a mixture of ε-caprolactam andhexamethylenediammonium adipate (nylon-6,6 salt).

High molecular weight, linear polyamide resins satisfactory for thepurposes of the invention include such high molecular weight resins asderived from the foregoing monomers and mixtures.

In U.S. Pat. No. 2,241,322 a cyclic amide such as ε-caprolactam andwater (the water content in the mixture varies from 1.6% to 61.5%) areheated under high pressure to a temperature between 180° to 300° C. toeffect a partial polymerization of the lactam. The pressure is thenreduced to atmospheric pressure and the water and unchanged monomergradually distilled from the mass and polymerization allowed to proceedto form a superpolyamide or fiber-forming resin. Where the initialpolymerization step is arrested and the mass is allowed to cool, theproduct will vary from a wax-like solid where the water content in themixture is about 1.6% to a cheese-like mass where the water is at anupper limit of about 61.5%. The polymer which is formed crystallizes ina dendritic-like structure and forms large aggregates of the dendriticstructures. These aggregates as formed are too coarse to permit directdispersion to form stable and useful dispersions. The cheese-like massesthat are formed from charges wherein the percentage of water is in theupper portion of the above stated range even when subjected to at least3 passes through a roll mill contain particles over a large size rangeup to particles as large as 1 mm.

In accordance with the present invention, a mixture of the polymerforming monomer or monomers, or a high molecular weight polyamide andwater is introduced into a pressure vessel or an autoclave that isprovided with means for agitating the mass during the heating period.The mixture or charge contains from about 30% to about 80%, preferablybetween 50% and 65%, by weight of the monomer or polyamide with thebalance water. If desired, the charge may also include an acidic oralkaline substance to serve to regulate the pH of the final quenchedmass. The pressure vessel is also provided with a dip tube extending toa position just above the bottom of the vessel to serve as a means fordischarging the mass at the termination of the heating period. The diptube is provided with a valve externally of the vessel and the externalend of the dip tube extends into a quench tank. After sealing thepressure vessel, the mass is heated to about 230° C. to 235° C. or otherapplicable elevated temperature and maintained at such temperature forfrom 4 to 24 hours, 6 to 10 hours being generally sufficient. Underthese conditions the pressure will rise to about 400 to 500 psi.

The higher the proportion of monomer or high polymer in the charge, thehigher the number average molecular weight of the polymer which is beingformed and the lower the proportion of monomer and oligomers; that is,polymer having a D. P. (degree of polymerization) not exceeding about 4.Roughly, using ε-caprolactam as the monomer, the number averagemolecular weight of the polymer formed and under the above statedconditions will vary between about 1300 and about 7,300, correspondingto a D. P. of between about 12 and about 65. The reduced viscosity asmeasured at 20° C. of a m-cresol solution containing 1 gm. of polymerper deciliter of solution will vary from about 0.1 to about 0.4.

In order to form the unique dispersible particles of the presentinvention, it is essential and critical that following the heating stepthe entire mass be quenched in an aqueous medium as rapidly as possibleto reduce the temperature of the mass below the freezing point of theformed polyamide, to continue the cooling of the mass so as to preventthe growth of the particles and prevent an alteration of the structureof the particles and to bring the entire mass to a uniform predeterminedpH value. In batch-wise procedure, quenching and cooling may be effectedin a suitable tank open to the atmosphere and provided with agitatingmeans, such as a Lightnin Mixer having a baffle plate, wherein thequench bath and discharging mass is subjected to good agitation so as toreduce the temperature of the discharging mass below about 90° C. to 95°C. as rapidly as feasible and to bring the entire mass to thepredetermined pH.

In batch-wise procedure, the valve on the dip tube is opened at thetermination of the heating period thereby allowing the autogenouspressure in the autoclave to force the reaction mass through the tubeinto the quench bath. The quench bath conveniently may consist of waterand crushed ice. Obviously, the quench bath may consist of water cooledby means of suitable cooling coils. The weight ratio of the quench bathto the reaction mass may vary from about 2:1 to about 6:1, generallybeing about 4:1. In those instances where the initial charge does notinclude a substance to provide the predetermined desired pH of the finalquenched mass, the pH controlling substance is incorporated in thequench bath. As the reaction mass issues from the dip tube into theagitated quench bath, the reaction mass is instantaneously brought to atemperature below the freezing point of the polymer, is cooled to atemperature sufficiently low so as to prevent particle growth andstructural alteration of the particles and brought to the predeterminedpH thereby forming uniformly sized particles of loosely packed, randomlyoriented, bonded lamellar sheets. In a typical example usingε-caprolactam and water as the reaction mass, such as will be describedin detail in Examples 1 to 9 set forth hereinafter, the total time todischarge 6 gallons of reaction mass and cool it to a temperature below90° C. was approximately 90 seconds, although, as stated, each incrementof the discharging mass as it leaves the dip tube becomes cooledinstantaneously.

Utilizing the foregoing conditions and the conditions as set forth inthe examples which are included hereinafter; that is, the composition ofthe initial charge and the reaction conditions of time, temperature, andpressure, the reaction mass attains approximate equilibrium composition.Obviously, the method will be practiced in a manner and under conditionsso as to produce a product having a predetermined desired molecularweight. Hence, as will be recognized, a higher proportion of monomer maybe included in the initial charge so that the desired molecular weightmay be attained in a shorter time period by arresting the reaction orpolymerization prior to reaching an equilibrium composition. In suchpractice, the quenched mass will contain higher proportions of monomerand oligomers than under equilibrium conditions and, advantageously, thequenched mass may be processed as by centrifugation or electrodepositionso as to separate the solids as a wet cake and recover a liquid phasecontaining some of the monomer and dissolved oligomers which may be usedin the preparation of subsequent charges.

The resulting product is a dispersion and the consistency will varydepending upon the solids content and the particle size of the polymerparticles will be dependent upon the pH of the original quenched mass.Where the particles of this invention do not exceed about 2 microns andthe solids content is in the useful range, the particles will remain insuspension or in a dispersed state for an indefinite period of time.Where the particles exceed about 2 microns and the pH of the dispersionis above about 6.5 some settling of the particles may take place withtime. However, the particles do not pack into a dense mass even overextended periods and they may be redispersed by merely shaking thecontainer or stirring the mass. The product may be used directly as acoating composition. In contrast to the prior art dispersions ofsuperpolyamides which do not dry to form self adherent coating onmetals, for example, the dispersions of the present invention whenapplied to a substrate such as a metal or glass and dried formself-adherent coatings on the substrate.

As stated hereinabove, a common proposal to form dispersions of highmolecular weight polyamides is to dissolve the resin in a solvent andprecipitate the resin in a nonsolvent as described in U.S. Pat. No.2,265,127. The resulting particles are usually of an irregular size andfibrillar in structure. In the partial degradation of high molecularweight polyamides followed by mechanical disintegration, the particlesare colloform or globular, granular in shape or structure and random insize. If the initial polymerization step using the higher proportions ofwater as described in U.S. Pat. No. 2,241,322 is arrested and the massis allowed to cool, the low molecular weight polyamide resin particlesare randomly sized and consist of densely packed, oriented lamellarsheets.

In contrast to these prior products, for example, the low molecularweight polycaprolactam resin particles of this invention, wherein thereaction mass is quenched rapidly and the pH of the quenched mass iscontrolled, the particles are uniform in size and consist of looselypacked, randomly oriented, bonded lamellar sheets or lamella or clustersof flaky sheets. When examined by the electron microscope, theseparticles appear much like a mass of wet cornflakes where the flakysheets drape over each other into loose clusters. It is believed thatthis flake-like structure aids in a spreading of the flakes over thesurface of the substrate in much the same manner as the so-called"metal" paints, that is, paints or coatings containing metal flakes suchas aluminum, bronze, etc.

The dispersion as resulting from the quenching of the reaction mass maybe used directly as a coating composition providing the quenched masspossesses the desired solids content. If desired, the solids may beseparated from the quenched mass in the form of a wet cake as bycentrifugation or electrodeposition so as to reduce the monomer andoligomers content and then redispersed in water to form a desiredconcentration of dispersed solids. Alternatively the wet cake may bewashed with water so as to remove additional monomer and oligomers whichmay be present in the wet cake prior to redispersing the solids inwater. The proportion of monomer and oligomers present in the quenchedmass will vary inversely with the proportion of monomer (assumingequilibrium conditions are used), or of high molecular weight polyamidein the initial charge. Inasmuch as the monomer becomes volatilizedduring subsequent heat treatment of the coatings, the monomer content ispreferably reduced by one or more of these processing steps particularlywhen non-equilibrium conditions are used in forming the polyamide. Inreference to the production of the polyamide from ε-caprolactam asoutlined above, the proportion of monomer and oligomers will vary fromabout 42% to about 14%, based on the solids content of the quenchedmass.

In the production of a dried product, the recovered quenched mass ispreferably spray dried and the monomer content will be lowered due tothe temperatures employed. If desired, both washing and spray drying maybe utilized to effect a reduction in the monomer and oligomers content.An outstanding characteristic of the spray dried products is the easewith which the product may be redispersed in water. A dispersion whichremains stable for extended periods of time may be formed by adding thespray dried powder to water and shaking the mass or by subjecting themass to agitation.

Since in most instances the completed coating desired is a highmolecular weight polyamide, a polymerization catalyst is preferablyincluded in the dispersion. Compounds which are or which upon heatingare converted to non-volatile, strong acids may be used as catalysts andinclude such compounds as orthophosphoric acid, monoammoniumorthophosphate, diammonium orthophosphate, orthophosphorous acid,metaphosphoric acid, p-toluenesulfonic acid and the ammonium salt ofbenzyl phosphite. In view of the range of pH of solutions of thesecatalysts, they may be advantageously incorporated in the initial chargeor in the quenching medium to control the pH of the final quenched mass.In general, the proportion of catalyst desired in the final coatingcomposition is between about 0.15% and about 0.8%, preferably 0.3% to0.6%, based on the weight of the polymer. Where lesser amounts are usedin the initial charge or quenching medium so as to obtain apredetermined desired pH, an additional amount of catalyst may be mixedinto the dispersion prior to its application to a substrate.

The aqueous dispersion, recovered quenched mass or reconstituteddispersion, is applied to a desired substrate by any conventionalmethod, such as brushing, dipping, spraying, electrostatic spraying,etc. In the subsequent heating step, water is evaporated and heating iscontinued so as to melt the polyamide flakes, allow the liquid phase toflow and permit molecular polymerization, thereby forming the adherentcontinuous coating. In the instance where the polymer has been preparedfrom ε-caprolactam, and the dispersion contains a catalyst, afterapplication of the coating to the substrate and the coating is subjectedto the required heat treatment, it has been found that there occursabout a 15-fold to about a 45-fold increase in the weight averagemolecular weight and the film properties are those of the usual highmolecular weight polycaprolactam or nylon-6 film. As an alternative, thedried powder product containing a catalyst may be applied to thesubstrate by any conventional powder coating technique, such as, forexample, fluidized bed coating, flame spraying, electrostatic spraying,etc. As a further alternative, the resin particles may be deposited froman aqueous dispersion on conducting substrates by electrophoretictechniques. The specific temperature utilized in the heat treatment willbe dependent upon the specific low molecular weight resin, generallybeing at the melting point of the resin. The period of heat treatmentwill vary directly with the thickness of the coating. Again referring toa low molecular weight polycaprolactam as produced as described herein,the coating will be heated to a temperature of about 235° C. for a shortperiod, such as 10 to 15 minutes where the coating thickness is betweenabout 2 and 5 mils.

The examples which follow illustrate the practice of the presentinvention but are not to be considered as limitations. Where referenceis made to percentages of various substances, the percentages are byweight unless stated otherwise. In the case of references to percentagesof additive or catalyst, the percentage is based upon the weight of theresin forming monomer or constituents. Stated pressures are pounds persquare inch (psi) gauge. Reduced viscosities were determined at 20° C.on m-cresol solutions of the resins containing 1 gm. of resin perdeciliter of solution.

EXAMPLES 1-9

In the preparation of the resins of these examples, a 10 gal.,electrically heated, stainless steel autoclave was utilized. Theautoclave was fitted with a stirrer and a dip tube having a valveexternally of the autoclave. The end of the dip tube internally of theautoclave extended to a position just above the bottom of the vessel.The external end of the dip tube was positioned near the bottom of a 30gal. stainless steel tank which was provided with a Lightnin Mixer. Ineach example, about 6 gals. of a solution of ε-caprolactam in distilledwater, after filtration to remove any possible foreign matter, with orwithout phosphoric acid, were charged into the autoclave and theautoclave sealed. The stirrer was operated at about 270 RPM. Heat wasapplied for the stated period of time. In general, 21/2 to 3 hours wererequired to bring the reaction mass to about 235° C. and the mass wasmaintained at this temperature to the end of the stated periods of time.The autogenous pressure reached about 460 to 500 psi. At the terminationof the heating period, heating was discontinued and the dip tube valveopened whereby the autogenous pressure in the autoclave forced the massthrough the dip tube into the quench tank. The quench tank contained amixture of about 160 lbs. of water and about 40 lbs. of crushed ice.Generally, the mass was discharged in about 90 seconds and thetemperature of the entire mass reduced to about 50° to 60° C. withinthis period of time. Specifically, in Example 6, 33.38 lbs. ofε-caprolactam was dissolved in 22.26 lbs. of distilled water. About 3hours were required to heat the mass to 235° C. and the pressure reachedabout 470 psi. The total heating period was 8 hours. Heating was thendiscontinued and the mass discharged in 90 seconds into a bath(temperature about 0° C.) consisting of 167 lbs. of water and 39 lbs. ofcrushed ice and at the end of the discharge period the temperature ofthe entire mass was about 51° C.

In this group of examples, the proportions of monomer, ε-caprolactam,and water varied from 30% monomer and 70% water to 80% monomer and 20%water. Samples of the dispersions (adjusted to about 14% to 15% solids)were poured into soft, thin gauge aluminum pans to provide coatings ofabout 5 mils in thickness. In those examples where no phosphoric acidcatalyst had been incorporated in the initial charge, 0.3% phosphoricacid (about 0.35% of 85% phosphoric acid) was added to and mixed intothe dispersion before pouring a sample into the aluminum pans. Thecoatings were then heated to 230° to 240° C. for about 15 minutes.Subsequently the coatings were stripped from the aluminum and thereduced viscosities of the film determined.

Samples of the dispersions were also tested in a thermobalance (PerkinElmer TGS-1). In this test a sample of known weight was first dried bypassing dry helium at room temperature over the sample until no changein weight was noted. The temperature of the sample was then increased ata rate of 10° C. per minute to 80° C. while continuing the flow ofhelium over the sample. The weight of the sample was noted and the lossin weight from the original sample was considered as the water contentof the dispersion. The heating rate was then continued until the samplehad been heated to 210° C. and the weight of the sample noted. The lossin weight of the sample between 80° C. and 210° C. was considered anapproximation of the monomer and oligomer content of the low molecularweight polymer produced. The properties of the products of this group ofexamples are tabulated in Table I. The molecular weight of the resinsand the film were determined by a combination of the gel permeationchromatography procedure and solution viscosity measurements. Samples ofthe dispersions were air dried and the melting points of the recoveredresins were determined by the use of the Fisher-Johns Melting PointApparatus.

                                      TABLE I                                     __________________________________________________________________________                      Reaction                                                                           Reduced      Resin                                                                              Reduced***                                        Additive                                                                           time viscosity                                                                          M-- .sub.w                                                                        M-- .sub.n                                                                        MP   viscosity                                                                           M-- .sub.w                                                                          Wt. loss                 Example                                                                            % CL                                                                              % H.sub.2 O                                                                       %*   hours                                                                              resin                                                                              (×10.sup.3)                                                                 (×10.sup.3)                                                                 ° C.                                                                        film  (×10.sup.3)                                                                   %                        __________________________________________________________________________    1    30  70  --   24   0.115                                                                              2.5 1.3 115-120  0.874                                                                           <60   41.5                     2    40  60  0.3  24   0.128                                                                              2.8 1.5 128-132  1.16                                                                            63.1  31.9                     3    50  50  0.3  8    0.160                                                                              3.0 1.6 150-153  1.20                                                                            64.6  24.2                     4    55  45  0.3  10   0.177                                                                              4.5 2.4 153-155  1.32                                                                            74.1  22.2                     5    60  40  --   8    0.200                                                                              5.2 2.7 **       1.32                                                                            74.1  22.0                     6    60  40  --   10   0.203                                                                              5.3 2.8 155-158  1.46                                                                            87.0  20.9                     7    60  40  --   24   0.199                                                                              **  **  **       **                                                                              **    **                       8    70  30  0.3  10   0.231                                                                              6.5 3.4 193-196  3.53                                                                            303   17.1                     9    80  20  0.3  10   0.397                                                                              13.8                                                                              7.3 197-200  5.04                                                                            <500  13.6                     __________________________________________________________________________     CL - ε-caprolactam                                                     *"Dash" indicates no additive used                                            **Not measured                                                                **After addition of 0.3% H.sub.3 PO.sub.4 to Examples 1, 5 and 6.       

EXAMPLES 10,11,12

In this group of examples, the equipment and general procedure asdescribed above were followed utilizing ε-caprolactam as the monomer andwater, however, no catalyst was added to either the initial charge orthe quench bath. The quantity of the quench bath was adjusted so as toprovide a final quenched mass containing approximately 14% solids. Theseexamples illustrate the reduction of monomer and oligomer content of theproduct by filtration, as by centrifugation, by washing with water andby spray drying. In each instance, the monomer and oligomer content ofthe quenched mass or product was determined as described above. Thequenched mass was then subjected to centrifugation in aKomline-Sanderson general purpose centrifuge, Model CL-10, fitted with asolid bowl attachment. The centrifuge was operated at 1800 RPM toprovide a filter cake of about 28% solids. The monomer and oligomercontent of the centrifuged mass was determined. The filter cakes werethen washed by redispersing the solids in water to provide dispersionscontaining about 14% solids and the dispersions again subjected tocentrifugation and the monomer and oligomer content measured. The filtercakes were again redispersed in water to form dispersions containingabout 14% solids and 85% phosphoric acid added in an amount sufficientto provide 0.3% phosphoric acid based on the weight of the solids. Thedispersions were subsequently spray dried by spraying the dispersions atroom temperature into a spray drying chamber, the introduced air havinga temperature of about 350° F. (177° C.) and the air leaving the chamberhaving a temperature of about 210° F. (99° C.). The spray dried powderscontained approximately 0.3% phosphoric acid thus exhibiting no loss ofphosphoric acid in the spray drying. Samples of the quenched masses, thewashed products and the spray dried products were tested in thethermobalance so as to determine the monomer and oligomer content of theproducts at the various stages. The properties were as reported in thefollowing table:

                                      TABLE II                                    __________________________________________________________________________                                    Oligomer & Monomer Content                              Reaction                                                                           Quenched mass    % based on solids                                       period    Final  Particle   Centrif.                                                                            Washed                                 CL/H.sub.2 O                                                                       Hours     Temp.  size Quenched                                                                            Quenched                                                                            &    Spray                        Example                                                                            %    235° C.                                                                     % Solids                                                                           ° C.                                                                       pH microns                                                                            Mass  Mass  Centrif.                                                                           Dried                        __________________________________________________________________________    10   50/50                                                                              10   14.1 53  8.43                                                                             5-6  29.3  9.8   7.7  4.4                          11   60/40                                                                              10   14.1 51  8.22                                                                             5-6  21.2  7.4   5.3  3.2                          12   70/30                                                                              5    14.1 49  8.07                                                                             5-7  19.6  4.8   2.4  1.7                          __________________________________________________________________________     CL - ε-caprolactam                                               

EXAMPLES 13-35

This group of examples illustrates the effect of the pH of the quenchedmass upon the size and the uniformity of size of the produced polyamideresin particles. The monomer used was ε-caprolactam. The equipment andpreparatory method used were as described in Examples 1-9. Thequantities of the various substances were selected so as to providequenched masses whose pH values varied over a pH range of between aboutpH 3 and about pH 9. The heating periods were varied and the maximumtemperature was about 235° C., except where noted. The particle sizes ofthe products were determined by microscopic examination. The reducedviscosities were determined as described above. In those instances whereno catalyst was present in the product as produced, 0.3% phosphoric acid(about 0.35% of 85% phosphoric acid) was added to a sample of theproduct before pouring the sample of the dispersion into an aluminum panfor the preparation of fused films. The properties of the products aretabulated in Table III which follows:

                                      TABLE III                                   __________________________________________________________________________              Additive   Quenched Mass                                                                              Particle                                              % &   Reaction  Final   Size                                        CL/H.sub.2 O                                                                            Location                                                                            time      Temp.   (mean)                                                                             Reduced Viscosity*                     Example                                                                            %    **    Hours                                                                              % Solids                                                                           ° C.                                                                        pH microns                                                                            Resin Film                             __________________________________________________________________________    13   60/40                                                                              --    8    12.8 54   8.58                                                                             5    0.2295                                                                              --                               14   60/40                                                                              --    8    12.4 53   8.52                                                                             5    0.2050                                                                              --                               15   60/40                                                                              --    8    14.2 37   8.48                                                                             4    0.2004                                                                              1.3240                           16   60/40                                                                              --    8    15.2 53   8.46                                                                             6    0.2390                                                                              --                               17   50/50                                                                              --    10   14.1 53   8.43                                                                             5    0.1876                                                                              --                               18   65/35                                                                              --     8-220°                                                                     13.2 40   8.32                                                                             3    0.2126                                                                              --                               19   65/35                                                                              --    8    13.3 40   8.32                                                                             6    0.2557                                                                              --                               20   60/40                                                                              --    10   14.1 51   8.22                                                                             6    0.2193                                                                              --                               21   60/40                                                                              0.43 (A)C                                                                           24   27.0 85   7.98                                                                             6    --    --                               22   50/50                                                                              0.3 (B)Q                                                                            10-265°                                                                     11.0 54   7.12                                                                             2    0.1632                                                                              --                               23   60/40                                                                              0.3 (B)Q                                                                            10   12.2 34   6.73                                                                             1    0.1862                                                                              1.5403                           24   40/60                                                                              0.3 (B)C                                                                            24   12.4 72   6.70                                                                             2    0.1281                                                                              1.1593                           25   50/50                                                                              0.3 (B)C                                                                            10   14.1 55   6.63                                                                             1    0.1596                                                                              1.2266                           26   70/30                                                                              0.3 (B)Q                                                                            10   14.0 36   6.61                                                                             2    0.2308                                                                              3.5315                           27   50/50                                                                              0.3 (B)C                                                                            10   18.5 76   6.57                                                                             1    0.1381                                                                              1.8160                           28   50/50                                                                              0.3 (B)C                                                                            8    12.7 53   6.53                                                                             1    0.1604                                                                              2.7475                           29   60/40                                                                              0.55 (B)Q                                                                           24   23.8 87   6.32                                                                             1    0.2305                                                                              --                               30   60/40                                                                              0.58 (B)Q                                                                           24   23.7 80   6.30                                                                             0.5  0.2256                                                                              1.8725                           31   60/40                                                                              7.7 (B)C                                                                            18   13.0 45   4.52                                                                             1    0.1835                                                                              0.6062                           32   60/40                                                                              7.7 (B)Q                                                                            18   13.0 45   3.58                                                                             1    0.2016                                                                              --                               33   70/30                                                                              1 NH.sub.4 St.C                                                                     5    13.4 45   8.88                                                                             <1   0.2528                                                                              --                               34   60/40                                                                              1 NH.sub.4 St.C                                                                     24   12.7 43   8.48                                                                             <0.1 0.2171                                                                              1.0812                           35   50/50                                                                              (1 NH.sub.4 St.C                                                                    10    3.7 43   3.05                                                                             <0.1 0.1576                                                                              --                                         (10 (B)Q                                                            __________________________________________________________________________     CL -- ε-caprolactam                                                   A -- Diammonium orthophosphate                                                B -- Orthophosphoric acid                                                     C -- Charge                                                                   Q -- Quench bath                                                              NH.sub.4 St. -- Ammonium stearate                                              *"Dash" indicates value not measured                                          *"Dash" indicates no additive used                                      

From the foregoing data it will be noted that, in the absence of a soapin the initial charge, the mean size of the resin particles are withinthe range of between about 2 microns and 6 microns in those instanceswhere the pH value of the quenched mass is within the range of about pH7 and about pH 9. Where the pH value of the quenched mass is within therange of about pH 3.5 and about pH 7, the mean size of the resinparticles are within the range of between about 0.5 micron and about 2microns. In the presence of a soap in the initial charge the mean sizeof the particles will be under 1 micron where the pH of the quenchedmass is within the range of about pH 3 and about pH 9. In Example 35where ammonium stearate was included in the initial charge andsufficient orthophosphoric acid was present in the quench bath to form aquenched mass of pH 3.05 the particles were of a size of approximately0.03 × 0.03 × 0.005 micron. Thus, conditions of preparation may becontrolled so as to provide particles of a size adapted for specificuses dictated by the desired thickness of a particular coating.

It will be noted that in Example 31, although the resin particles asformed possessed a low molecular weight as measured by the reducedviscosity, the fused film did not exhibit a typical increase inmolecular weight. Failure to attain the high molecular weight was due tothe presence of an amount of orthophosphoric acid (7.7%) beyond theuseful range for catalyzing a low molecular weight polycaprolactam tothe desired high molecular weight.

The application of the present invention to the preparation of otherpolyamides from other monomers and the preparation of finely-dividedparticles of copolymers is illustrated by the examples which follow:

EXAMPLE 36 and 36A

Hexamethylenediammonium adipate (nylon 6,6 monomer salt) was prepared asdescribed in Preparative Methods of Polymer Chemistry, 2nd edition, byWayne R. Sorenson and Tod W. Campbell, page 74. The preparation of lowmolecular weight resin particles followed the general procedure asdescribed above. In this example, a 2 liter stainless steel laboratoryautoclave was fitted with a stirrer and dip tube in a manner asdescribed above. In each instance, a 1080 gram mixture of the monomersalt and water was used. In the first case the mixture contained 60% ofthe monomer salt, while in the second case the mixture contained 80% ofthe monomer salt. In each case, after introducing the mixture into theautoclave, the autoclave was sealed and heat was applied for 24 hours.The maximum temperature was 280° C. In the first case, the reaction masswas discharged into an agitated quench bath consisting of 1620 grams ofwater and 1620 grams of crushed ice while in the second case, 2340 gramsof water and 2340 grams of crushed ice were used. Both quenched masseswere creamy dispersions, the first containing 11.6% solids and having apH of 9.4, while the second contained 12.1% solids and had a pH of 9.3.The particles of the first product had a bimodal distribution of sizeswith two peaks; that is, one group of particles having a mean size of 2microns with the other group having sizes between 10 and 60 microns. Themean size of the particles of the second product was about 15 microns.

EXAMPLES 37-40

In this group of examples, copolymers were prepared from mixturesconsisting of 85% ε-caprolactam (nylon 6 monomer) with 15%hexamethylenediammonium adipate (nylon 6,6 salt), or 15%amino-undecanoic acid (nylon 11 monomer) or 15% ω-lauryl lactam (nylon12 monomer). The general preparatory procedures were as describedhereinbefore. In Examples 37,38 and 39, the products were preparedutilizing the 2 liter stainless steel autoclave, while in Example 40,the 10 gal. autoclave was utilized. After introducing a mixture ofmonomers and water into the autoclave and sealing the autoclave, heatwas applied for the stated period. In each example, the maximumtemperature was about 235° C. After the heating period, the reactionmass was discharged into an agitated quench bath consisting of water andcrushed ice. The properties of the products were as tabulated in TableIV which follows:

                                      TABLE IV                                    __________________________________________________________________________                                  H.sub.3 PO.sub.4   Particle                                                                               Resin               M/H.sub.2 O                   Quench                                                                             Time                                                                              Quenched Mass                                                                           Microns  MP                  Example                                                                            %    CL     M2    H.sub.2 O                                                                            Bath Hours                                                                             % Solids                                                                            pH  Mean Max °            __________________________________________________________________________                                                              C                   37   50/50                                                                              342 g  198 g 540 g  none 24  21.7  8.58                                                                              1    2    95-100                              (N66)                                                        38   60/40                                                                              551 g  97 g  432 g  0.6% 18  10    5.68                                                                              0.5  1   115-120                              (N11)                                                        39   50/50                                                                              459 g  81 g  540 g  0.3% 10  12    6.48                                                                              1    1   105-110                              (N12)                                                        40   50/50                                                                              23.65 lbs                                                                            4.17 lbs                                                                            27.82 lbs                                                                            0.3% 10  8.2   6.62                                                                              1    1   105-110                              (N12)                                                        __________________________________________________________________________     M - Monomer mixture                                                           CL - ε-caprolactam                                                    M2 - Comonomer                                                                N66 - Nylon-6,6 salt                                                          N11 - nylon-11 monomer                                                        N12 - Nylon-12 monomer                                                   

Finely-divided, water dispersible, low molecular weight polyamideparticles as described herein may be produced from high molecular weight(fiber- and film-forming) polyamides, such as scrap or waste fibers,film scrap, etc., as illustrated by the following example:

EXAMPLE 41

The high molecular weight polyamide utilized was a commercialpolycaprolactam product marketed as Plaskon 8200 Nylon 6 MoldingPellets, Natural Grade (Allied Chemical Co.). The pellets, as received,were ground in a Wiley Mill to pass a 20 mesh screen (841 micronsopenings). A charge consisting of 500 gms. of the groundpolycaprolactam, 500 gms. of water and 5 gms. of ammonium stearate wasintroduced into the 2 liter autoclave. After sealing, heat was appliedfor 35 minutes, the maximum temperature reaching 235° C. The heated masswas discharged into an agitated quench bath consisting of 1400 gms. ofwater, 1400 gms. of crushed ice and 2.75 gms. of 85% H₃ PO₄. Thequenched mass had a temperature of 50° C., a pH of 6.72 and contained13% solids. The particles had a size of 1 to 3 microns. The reducedviscosity of the original polycaprolactam was 2.6670. The product had areduced viscosity of 0.2533. A sample of the quenched mass afterconversion to a fused film in a manner as described above exhibited areduced viscosity of 1.5280.

Aqueous dispersions of the finely-divided, water dispersible, lowmolecular weight, linear polyamide particles, either as the formedquench mass, or after concentration to a desired solids content, orafter washing to reduce the monomer and oligomer content and redispersedto a desired concentration, or after redispersing a spray dried product,may be applied to a desired substrate. Obviously, in those applicationsto substrates, such as, for example, metallic substrates, glassstructures, such as, glass fibers, etc., where it is desired to convertthe coating to a high molecular weight polyamide, the dispersion shouldcontain a polymerization catalyst as set forth hereinbefore. Where thedispersion is intended for uses not requiring a high molecular weightpolyamide as desired in the coating art, a polymerization catalyst maybe omitted. For example, where the low molecular weight polyamide isintended as a cross-linking or curing agent for water-dispersible epoxyresin compositions, the polyamide catalyst may be omitted.

It is obvious that the dispersions may contain a variety of additives,such as, for example, coloring materials (dyes, pigments, etc.), thermalstabilizers, antioxidants, ultra-violet light stabilizers and biocides.As is obvious, certain of the additives which are non-reactive with thepolyamide or monomer and are not affected by the temperatures involvedin the heating step may be incorporated in the initial charge. Where theadditive is heat sensitive, it may be added to the quench bath or to thedispersion prior to its application to the substrate.

The application of the coatings to metallic substrates may beillustrated by the examples which follow.

EXAMPLE 42

A portion of the quenched mass of Example 15 was centrifuged and thefilter cake dispersed in water to form a dispersion containingapproximately 25% solids. Sufficient orthophosphoric acid was mixed withthe dispersion to provide 0.3% H₃ PO₄, based on the solids content. Themetallic substrates were 3 × 6 inches, 24 gauge, cold rolled steelpanels with Bonderite 37 treatment. The panels were provided with a 10mil wet coating of the dispersion by means of a doctor blade. The panelswere placed in an oven through which nitrogen was circulated and heatedto 230° to 240° C. for 15 minutes. The finished coatings wereapproximately 2 mils in thickness.

Coated panels were subjected to an impact test both directly (oncoating) and indirectly (on reverse side of panel) in a Gardner HeavyDuty Impact Tester, Model lG-1120, having 4 pound weight terminating ina 5/8 inch ball head, the weight being dropped from a 40 inch height.The coatings withstood the maximum impact; namely, a 160 inch-poundimpact, both direct and indirect without separation from the panels norwas there any evidence of a cracking or crazing of the coatings.

Coated panels were also subjected to a flexibility test by the use of aGardner Mandrel Set, Model MG 1410, commonly employed in the testing ofpaint coatings. In this test, the coated panel is bent around a 1/8 inchdiameter rod, the uncoated side being in contact with the rod. In thistest, the coatings exhibited no separation from the panels and nocracking or crazing of the coatings.

In a third test, known as the "3M Scotch Tape Cross-Hatch AdhesionTest", a grid is scored through the coating to the metal with a knifeedge, the grid consisting of 11 × 11 lines, the lines being spaced 1/16inch. 3M Scotch Tape, approximately 3/4 inch in width, is then appliedover the area of the grid and rubbed so as to effect good adhesion overthe area of the grid, leaving an unadhered portion of the tape beyondthe area of the grid. The unadhered tab is then grasped and the tape ispulled off rapidly. The results of this test are expressed in the numberof the 1/16 × 1/16 inch areas of the coating which are removed. In suchtests of the coated panels, no areas of the coating were removed.

For the production of finish decorative coatings where a plane surfaceof high smoothness and a high degree of uniformity in thickness isdesired, that is, a surface free of minute depressions or surfacecraters and free of an orange-peel effect and the like, a flow promotermay be included in the dispersion. The flow promoter may be included inthe quench bath, or may be added to the dispersion before spray drying,or may be added to a dispersion prior to its application to thesubstrate. The amount of flow promoter may vary from about 1% to about8%, preferably 3% to 5%, by weight, based upon the solids content of thedispersion. Flow promoters satisfactory include n-butylurea, nylon 6,6salt, nylon 6,9 salt, nylon 6,10 salt, intermediate molecular weight,water soluble polyethyleneimines, such as the commercial productmarketed as NC-1612 by Dow Chemical Co., and water emulsifiable epoxyresins, such as the commercial modified bisphenol Aepichlorohydrin-based epoxy resin marketed as Genepoxy M205 by GeneralMills Chemicals, Inc.

In the electrophoretic coating of conducting surfaces, the polyamideparticles may be provided with either a positive charge whereby theparticles will be deposited on a cathode, or with a negative chargewhereby the particles will be deposited on an anode. Thus where thedispersion has an acidic pH, as resulting from the presence ofphosphoric acid catalyst, for example, the particles will be depositedon the cathode, whereas, if the dispersion has a basic pH, as resultingfrom the presence of diammonium phosphate catalyst, the particles willbe deposited on the anode. The electrophoretic coating may beillustrated by the following example:

EXAMPLE 43

A portion of the spray dried product of Example 14 was dispersed inwater by means of a Waring Blendor to form a 5% solids dispersion.Sufficient orthophosphoric acid was added to provide 0.6% H₃ PO₄ basedon the solids content. A sufficient amount of a 50% water emulsion ofGenEpoxy M205 was added to provide 2% of the flow promoter based on thesolids content. The pH of the final dispersion was 6.7. The dispersionwas then transferred to a stainless steel tank which was subsequentlymade the anode. The cathode consisted of aluminum alloy (Gardner PG1304A) panels, 3 × 6 inches × 20 mils. The panels were immersed in thedispersion and 50 V.D.C. applied for approximately 10 seconds. Thepanels were subsequently placed in an oven through which nitrogen wascirculated and heated to 230° to 240° C. for 10 minutes. The coatingthus formed had a thickness of approximately 1 mil. The continuity ofthe coatings was tested by the use of a 2.2% hydrochloric acid solutioncontaining about 1% copper sulfate, a copper deposit being indicative ofa "pin hole" in the coating. Areas of the coating were covered withdrops of the acidic solution and observations were made by the use of amicroscope covering a period of 10 minutes. No copper deposits wereobserved, thus indicating the coatings to be free of "pin holes."

The electrostatic powder spraying of the low molecular weight polyamideproduce is illustrated by the following example:

EXAMPLE 44

Example 16 was repeated to provide a dispersion of the low molecularweight polycaprolactam particles. Sufficient 85% orthophosphoric acidwas added to the quenched mass so as to provide 0.6% H₃ PO₄, by weight,based on the weight of the polycaprolactam. A sufficient amount of a 50%water emulsion of GenEpoxy M205 was added to provide approximately 3%,by weight, based on the polycaprolactam, of the flow promoter. Themixture was blended for about 15 minutes by use of a Lightnin Mixer. Theresulting aqueous dispersion was spray dried by spraying the dispersionat room temperature into a spray drying chamber, the introduced airhaving a temperature of about 350° F. (177° C.) and the air leaving thechamber having a temperature of about 210° F. (99° C.). The spray driedproduct recovered was free flowing and had a mean particle size ofapproximately 20 microns.

The metallic substrates were as described in Example 42. Conventionalelectrostatic powder spraying apparatus was used wherein the steelpanels were grounded. The spray dried powder was blown through the spraygun where the particles were given in a high-voltage low-amperagenegative charge as they left the spray gun and thus were attracted toand deposited on the grounded panels. The coated panels weresubsequently heated in an air oven to about 205° C. for 10 minutes. Theresulting coating was approximately 2 mils in thickness. The coatingswere subjected to the "3M Scotch Tape Cross-Hatch Adhesion Test" asdescribed in Example 42. In such tests, no areas of the coating wereremoved.

In the foregoing example reference is made to the use of a spray dried,free flowing powder having a mean particle size of about 20 microns. Theunique characteristic of the spray dried products is that when such aproduct is added to an aqueous medium and the mixture is subjected toagitation as by use of a Lightnin Mixer or Cowles Dissolver, the productreverts to the approximate particle size of the material before spraydrying and the resulting dispersion is almost indistinguishable from thedispersion from which the dry powder was derived.

Glass fibers and filaments have a harsh hand or feel and poor resistanceto abrasion when rubbed together and necessitate specialized handling toconvert them into textile products. Because of the non-hydrophilicnature of glass, the conventional yarn finishes have not beensatisfactory. The coatings formed from dispersions of the low molecularweight polyamides of the present invention overcome these inherentdisadvantageous characteristics. In the production of glass filaments,they may be passed over or between rolls so as to apply a dispersion byroller coating. The thickness of the coating may be controlled by thesolids content of the dispersion and by the particle size of thedispersed polyamide particles. The filaments are then passed through asuitable heating zone so as to fuse the coating. The coating isflexible, tough and is resistant to abrasion and permits the use of yarnfinishes as conventionally utilized in the nylon textile industry.

Furthermore, as indicated hereinbefore, the polyamide dispersion maycontain coloring materials such as pigments or dyes so as to provideglass based filaments of any desired color. Alternatively, the coatedfilaments or textile products formed from such filaments may be dyed toa desired color.

Although in some of the preceding examples and in the foregoingdiscussion the dispersions of the polyamide particles have been applieddirectly to a substrate, in certain instances it may be desired to applyto a substrate a different coating or polymer which may not have asufficiently high toughness and/or abrasion resistance. In suchinstances, the use of the present dispersions may be advantageously usedto form an overcoat of high toughness and abrasion resistance. Forexample, in the wire coating art, in many instances, the wire may firstbe provided with an enamel coat such as a polyester coat deposited froma solution of the polyester. The dispersions of the present inventionmay be applied over the base coat and cured as described above toprovide an outer coating of greater toughness necessary for subsequentwinding operations.

In the illustrative examples, the coatings after application to asubstrate were heated to temperatures between about 205° C. The specifictemperature used to melt the low molecular weight polyamide particlesand permit the molten material to flow and to polymerize the polymer toa desired high molecular weight must be, obviously, at least the meltingpoint of the specific low molecular weight polyamide. Highertemperatures may be used so as to reduce the require polymerizationperiod provided that the temperature is not sufficiently high toadversely affect the polymer, as by decomposition.

In the foregoing discussion, reference has been made to"superpolyamides" and "fiber- and film-forming polyamides" and theseterms have been used in the sense first enunciated by Carothers. Thesimple test usually used to define fiber-forming polymers has involveddipping an end of a rod into the molten polymer and withdrawing the rodso as to determine whether or not a self-supporting filament could bedrawn from the molten polymer. As indicated by Carothers, fiber-formingpolymers (superpolymers) generally require a molecular weight of atleast 10,000 for minimum fiber properties. The term "superpolymer" wascoined by Carothers to describe polymers having molecular weight above10,000 (Textbook of Polymer Science, 2nd Ed., 1971, by Fred W.Billmeyer, Jr.). For practical fiber-forming purposes the molecularweight should substantially exceed 10,000. For example, in Encyclopediaof Polymer Science and Technology, 1st Edition, in the section entitledPolyamides by W. Sweeny and J. Zimmerman (page 542), it is pointed outthat commercial nylon fiber has a number average molecular weight offrom about 12,000 to 15,000.

The term "low molecular weight polyamide" as used herein and in theclaims is intended to designate polyamides having a number averagemolecular weight of from about 1,300 to not exceeding about 7,300. Thepolyamide products of the present invention preferably have a reducedviscosity within the range of from about 0.15 to about 0.26. As isapparent from the foregoing discussion the molecular weight or reducedviscosity may be controlled by the relative weight proportions of thepolyamide forming constituent and the water. The specific particle sizeand uniformity of particle sizes in a specific product is controlled byan instantaneous quenching of the heated reaction mass and the pH of thequenched mass.

It is obvious that the examples illustrate the preparation of theproducts by batch procedures. Conveniently, in such preparations theinstantaneous cooling of the reaction mass is effected by dischargingthe reaction mass into an agitated quench bath. It is obvious that othermeans may be utilized, particularly in a continuous method. For example,the quenching medium may be pumped through a conduit positioned so thatthe quenching medium impinges on or collides with the reaction mass asit is discharged from the dip tubes and the resulting quenches mass thencollected in a suitable vessel. Alternatively, the dip tube maydischarge the reaction mass and the quenching medium may besimultaneously pumped into a mixing chamber from which the quenched maybe withdrawn continuously. In the latter procedures, any desiredadditive, such as catalysts, flow promoters, stabilizers, pigments,etc., may be conveniently metered into the conduit through which thequenching medium passes so as to aid in providing an intimate mixture orblend of these agents throughout the quenched mass.

As stated hereinabove, one of the unique characteristics of thewater-dispersed polyamide particles is the very narrow size distributionof the particles. In general, the size distribution of the particles ofany specific product will vary directly with the means particle size;that is, the lower the mean particle size, the narrower the sizedistribution. The particle size distribution is about d ± 0.8d, where dis the mean particle size; in other words, the sizes of the particles inany specific product will be within a range of between about d - 0.8dand about d + 0.8d. Thus in Example 35 where soap was present in theinitial charge and the quenched mass has a pH of about 3, the meanparticle size was approximately 0.03 × 0.03 × 0.005 micron and theparticles ranged in the larger dimension from about 0.006 micron toabout 0.054 micron.

Upon spray drying of a dispersion of the polyamide resin particles, theparticles become bound together loosely into agglomerates and form afree flowing powdery product. The size of the dry, free flowing powderparticles may be within a range of between about 3 microns and about 150microns, the mean particle size of any specific product being dictatedby the intended use. The unique characteristic of these loosely boundagglomerates is that when subjected to agitation in water, theagglomerates readily disintegrated of crumble into what appears to bethe same particles as were present in the original dispersion prior tospray drying. For example, when the dispersions of Examples 14, 18 and19 were spray dried, the dried products were free flowing and consistedof agglomerates of sizes within the range of 3 microns to about 35microns. Upon agitation in water by the use of a Cowles Dissolver, theagglomerates had broken up into dispersed particles having mean sizes ofabout 5-6 microns, 3-6 microns and 5-6 microns, respectively. Where thepowder product is intended for use in electrostatic and flame sprayingtechniques, the mean particle sizes are preferably in the lower portionof the range, for example, between 20 microns and 40 microns. Where theproduct is intended for fluidized bed techniques, coarser particles arepreferred such as products having mean particle sizes between about 75microns and 150 microns.

The particles as formed from ε-caprolactam and copolymer predominatingin ε-caprolactam are loosely packed, randomly oriented, bonded lamellarsheets, or clusters of flaky sheets which are plate-like in structurewith two dimensions roughly equal and having irregular or crenulatededges. In the case of the particles produced from the nylon-6,6 monomersalt (Example 36), the ultimate particles are similarly lamellar sheetsbut are more bladed-to-fibrous in structure.

In the foregoing description and discussion, references have been madeto particle sizes. In Example 35, the particles are ultimate particlesand consists of flaky sheets or lamella, the particle size (d)designating the maximum dimension of the lamella. The particles, forexample, as formed in Examples 13-32, consist of loosely packed clustersof flaky sheets and the particle size (d) designates the maximumdimension of approximate diameter of the clusters. The spray driedproducts consist of loose agglomerates of the clusters of flaky sheetsand, as stated above, readily separate into the original clusters offlaky sheets when subjected to agitation in water. The particle size ofthe spray dried products has reference to the maximum dimension ordiameter of the agglomerates of the clusters. Aggregates designatematerials which consist of tightly bonded particles which do notseparate into the original particles when subjected to agitation inwater.

In the production of spray dried products, the dispersion which is to bespray dried should not contain ultimate particles such as produced whensoap is included in the initial charge. The presence of such submicronparticles results in the formation of aggregates wherein the particlesare tightly bound and up agitation in water the aggregates do notdisintegrate into the original particles. It is essential that thedispersion to be spray dried contains particles consisting of looselypacked, randomly oriented, clusters of flaky sheets so that the driedproduct consists of loosely bound agglomerates which disintegratereadily upon agitation in water to form the particles as were present inthe dispersion prior to spray drying.

However, if desired, spray dried products may be prepared from the soapcontaining dispersions (Examples 33-35) or form dispersions containingsub-micron particles. Such spray dried products will consist ofaggregates that are capable of withstanding abrasion such as occurs influidized bed coating techniques. In such techniques where the coatingpowders are in a continuing swirling motion, the aggregates because theycontain the original particles tightly bonded together do not crumble orbreak apart into minute particles which will escape or be blown from thefluidized bed chamber. The aggregates, however, will deposit on theheated substrate, melt and flow and the polyamide may be polymerized asdescribed above.

What is claimed is:
 1. As an article of manufacture, free flowing, spraydried low molecular weight, linear polyamide resin powder having areduced viscosity of between about 0.1 and about 0.4, consistingessentially of loosely bound agglomerates of loosely packed, randomlyoriented, clusters of flaky sheets, the flaky sheets having a meanparticle size between 0.03 micron and about 6 microns and the particlesize distribution is about d±0.8d where d is the means particle size,the agglomerates having a size within a range of between about 3 micronsand about 150 microns and being further characterized in that whendispersed in water the agglomerates disintegrate into loosely packed,randomly oriented, clusters to flaky sheets having particle sizes ofapproximately the same size and size distribution as the original flakysheets and form a stable dispersion of flaky sheets.
 2. The article ofmanufacture as defined in claim 1 wherein the polyamide resin isselected from the group consisting of ε-caprolactam, a copolymer ofε-caprolactam and 11-aminoundecanoic acid, a copolymer of ε-caprolactamand 12-aminododecanoic acid and a copolymer of ε-caprolactam andhexamethylenediammonium adipate, the polyamide resin having a reducedviscosity between about 0.15 and about 0.26.
 3. The article ofmanufacture as defined in claim 2 wherein the free flowing powderincludes between about 0.15% and about 0.8%, base on the weight of thepolyamide resin, of a catalyst consisting of a non-volatile, strong acidor a compound which upon heating is converted to a non-volatile strongacid.
 4. The article of manufacture as defined in claim 3 wherein thefree flowing powder includes between about 1% and about 8%, based on theweight of the polyamide resin, of a flow promoter.
 5. The method offorming free-flowing, low molecular weight, linear polyamide resinpowder which comprises mixing a least one polyamide forming monomer orhigh molecular weight, linear polyamide and water, heating the mixtureunder pressure to a temperature sufficiently high and for a periodsufficient to form a linear polyamide of a predetermined low molecularweight, the temperature exceeding the melting point of the formedpolyamide, thereafter instantaneously quenching the heated mixture in anaqueous quenching medium to instantaneously reduce the temperature ofthe heated mixture to a temperature below the freezing point of theformed polyamide and to bring the mass to a pH between about pH₃ and pH₉thereby converting the formed polyamide into finely-divided, waterdispersible, low molecular weight, linear polyamide resin particleshaving particle sizes within a narrow particle size distribution and toinstantaneously reduce the temperature of the mass to a temperaturewhere particle growth is prevented and the particle size and structureis retained, recovering a stable aqueous dispersion of thefinely-divided, low molecular weight, linear polyamide resin particlesin the form of loosely packed, randomly oriented clusters of flakysheets in the aqueous medium, spray drying the recovered stable aqueousdispersion and recovering a free-flowing, low molecular weight, linearpolyamide resin powder consisting essentially of loosely boundagglomerates of loosely packed, randomly oriented, clusters of flakysheets.
 6. The method as defined in claim 5 wherein the mixture containsbetween about 30% and about 80% by weight of the monomer or polyamidewith the balance water, the mixture is heated to a temperature betweenabout 220° C. and about 280° C. for from about 4 hours to about 24 hoursand the heated mixture is instantaneously quenched to reduce thetemperature of the heated mixture to a temperature below about 95° C.